The invention relates to a system and method for concentrating an aqueous beverage and to a separator useful therein as well as apart therefrom.
Concentrating aqueous beverages for storage, transportation and sale has a number of substantial advantages and is being used more and more widely for an increasing variety of beverages. For some products, for example, coffee and tea, the purpose of concentration is to produce a product which is convenient for the consumer to use. For other products such as wine, milk, beer, vinegar and the like, the greatest advantage may lie in reducing the bulk of the material and thus reducing the expense of storage and transportation.
Concentration of such aqueous beverages can be done in one of three ways--evaporation, freeze concentration, or reverse osmosis. In evaporation techniques, the beverage is heated or steam is passed therethrough to remove the water by evaporation. In freeze concentration techniques, a slurry of ice is formed in the beverage and the ice is then separated from the resulting concentrated liquor. One of the drawbacks to evaporation techniques is that many of the subtle flavor components of aqueous beverages are volatile and escape during evaporation. This difficulty can in part be overcome by stripping many of those components before evaporation and then returning them to the concentrated beverage. However, some degradation in flavor seems to be inevitable with evaporation techniques. Reverse osmosis is non-selective and flavor components are lost making it unsatisfactory for concentration of aqueous beverages.
Freeze concentrated products do not suffer from degradation since retention of flavor components is almost one hundred percent. The main drawbacks in the past to freeze concentration processes have been expense and insufficient volume of operation.
The present invention relates to a process and system for freeze concentration which is more efficient than previous techniques and which can process large volumes of concentrated aqueous beverages in relatively short periods of time.
The patent to Thijssen et al U.S. Pat. No. 4,004,886 describes a process and apparatus for crystallization in which a slurry of seed ice crystals and mother liquor are produced in a scraped surface heat exchanger and continuously supplied to a recrystallization vessel in which the crystals grow. The mother liquor in the recrystallizer vessel is continuously mixed and recirculated to the scraped surface heat exchanger via a filter which prevents crystals from leaving the recrystallizer. Almost all of the crystals in the recrystallizer melt and reform on a few small seed crystals to produce a crystal slurry having relatively uniform sized crystals therein, which slurry is removed continuously from the recrystallizer as a crystal suspension. In this arrangement, the residence time in the system is substantially reduced because of the melting of the seed crystals supplied to the recrystallizer from the heat exchanger and reformation of these melted crystals onto the few large crystals which then grow as spheres. While others in the past have proposed systems using both scraped surface heat exchangers and larger tanks in which crystal growth takes place, for example, the patent to Walker U.S. Pat. No. 3,156,571, it is the melting of the vast majority of the ice crystals and the recirculation only of liquid from the recrystallizer which reduces the residence time and produces the uniform crystal size in both the system described in the Thijssen et al patent, and the present invention. The uniform crystal size in particular permits use of wash columns in the system instead of centrifuge or other separating devices which have technical and other disadvantages.
The slurry which is removed from the recrystallizer in the above-described system of the Thijssen et al patent is preferably supplied to a wash column, for example, as described in the Thijssen U.S. Pat. No. 3,872,009. In this particular wash column, the slurry is supplied to the bottom of a column and then compacted against the ice mass by a piston which periodically pushes the mass upward. The ice at the top of the column is chopped and removed from the column where it is melted and at least in part returned to the column to flow downward to maintain a washfront when the piston applies pressure to the bottom of the column. The mother liquor is removed as concentrated liquor through perforations in the piston.
In a system as described in our U.S. Pat. No. 4,316,368, a plurality of concentrating units are connected together for counter-current operation. Countercurrent freeze concentration as such is not new. For example, Ganiaris U.S. Pat. No. 3,283,522 describes a multi-stage freeze concentrating system in which ice passes toward the first stage and mother liquor toward the last stage. However, in the system of our patent, only the crystals from the succeeding stage grow; in all stages except the last, practically all seed crystals (of the order of 99% and at least more than 90%) formed in that stage melt and reform on the larger crystals from the succeeding stages and this remarkably improves the efficiency of concentration. Further, the separation is done in the lowest concentration step where viscosity is lowest and the wash column performs most efficiently.
In the first stage which receives the feed liquid to be concentrated and produces a first intermediate concentrated solution, a slurry of ice crystals and liquid in a recrystallization vessel are supplied to a separator such as a wash column and the intermediate concentrated solution is passed to a second stage. The ice crystals from the second stage are separated from at least part of the slurry liquid and passed countercurrent to the direction of movement of the aqueous beverage liquid and to the recrystallization vessel of the first stage. Almost all of the seed crystals produced in the first stage, for example, by a scraped surface heat exchanger, then melt and reform upon the larger crystals supplied from the second stage. Third and additional stages can also be provided, each passing at least the ice back directly to the recrystallization vessel of the preceding stage so that the crystalline growth takes place only on the crystals which are supplied from the succeeding stage and practically all of the crystals generated in each stage except the last melt and reform thereon.
By utilizing this countercurrent approach, three stages which each can remove 250 kilograms of ice per hour from a liquid feed will remove at least 1800 kilograms per hour in a countercurrent configuration, as opposed to 750 kilograms per hour in parallel operation and 1200 kilograms per hour in serial operation in which only the liquid is passed through succeeding stages.
The water removal capacity in kilograms of ice per hour of any freeze concentration system depends on the viscosity of a given product concentration and the diameter of the ice crystals at that concentration. The viscosity of any liquid is strongly dependent upon its concentration. The crystal growth velocity is dependent also upon concentration so that an increase in concentration results in a sharp decrease of the crystal growth velocity and an increase in viscosity, both of which substantially reduce the rate of crystal growth. Using the countercurrent approach, crystal growth can take place on crystals which have already grown large and can take place in a less concentrated solution, both factors decreasing residence time and hence increasing capacity. Separation in the lowest concentration stage is also most efficient.
Efficient separation of solids from liquids in a slurry or the like is, in general, difficult, particularly in the system of our previous patent. The apertures through which liquid must pass are subject to clogging and often such devices are subject to frequent breakdowns. Viscous slurries are particularly difficult to separate.
The present invention utilizes structure which is mechanically simple and reliable, which operates under a variety of conditions, which handles viscous slurries and which finds particular utility in the system described in our previous patent. The separator includes a vessel having an interior space, an inlet into that space through which the slurry to be separated enters the vessel, a first outlet for the solids, and a second outlet for at least some of the slurry liquid. A cylindrical filter is rotatably mounted within the vessel in the space between the inlet and second outlet so that liquid flows through the filter to the second outlet while the solids accumulate thereon. The solids are scraped off the filter and pass out the first outlet. In one embodiment the solids are slurried with liquid injected from a second inlet whereas in a second embodiment a part of the feed liquid slurries the solids.
The first inlet in the second embodiment, and the only inlet in the first embodiment, can be positioned so that the slurry enters the vessel parallel to the axis of rotation of the filter and the liquid exits parallel thereto or so that the slurry enters perpendicular thereto and the liquid exits through an outlet positioned opposite the inlet.
This unique separator is particularly efficient in separating the crystals from most of the liquid and for returning the crystals to the preceding stage as described in our patent. However, the separator can also be used to improve the efficiency of other parts of the system of our earlier patent.
First, the end concentration is limited by the maximum viscosity at freezing point which can be handled by the filter of the recrystallizing vessel. The separator described above can separate effectively at high viscosity. Therefore, final product concentration can be increased by adding a scraped surface heat exchanger and separator to the final stage where the product liquid is removed. The heat exchanger can replace one of the other heat exchangers of the system. The filtered liquid removed from the final stage recrystallizer which hitherto was the final product is instead supplied to the scraped surface heat exchanger where a part of the water is frozen to tiny ice crystals and the viscosity increased. The slurry is injected into the separator where a part of the liquid is withdrawn as product flow. The slurry with the increased ice content is then fed back to the final stage recrystallizer.
Second, the separator can be effectively used to filter the flow from the recrystallizer to the scraped surface heat exchangers. The separator can be mounted internal to the recrystallizer vessel and the filter rotated by the stirrer motor. In bigger recrystallizer vessels the shaft for the filter may have to be too long to be so driven. In this case separate driven internal filters can be applied. If the desired filter surface cannot be built in the recyrstallizer vessel, the separator can be external to the recrystallizer vessel. This has the additional advantage that one separator can be provided for each scraped surface heat exchanger.
Other purposes and objects of the invention will be clear from the following detailed description of the drawings.